Building tack is a term of art employed in rubber processing technology. It generally refers to the surface property of any compounded unvulcanized rubber stock which enables two pieces of unvulcanized rubber to adhere together when brought in contact under moderate pressure. It is generally measured by the amount of force required to separate two pieces of such rubber stock after they have adhered together for a short period of time. A convenient method of evaluating building tack is by the use of a device called Ketjen Tackmeter. Tack is an important and necessary property of various compounded rubber stocks in an uncured state in order that these uncured compounded rubber stocks may be fabricated into various commercially useful rubber products, such as tires, hoses, belts and the like. A problem is sometimes encountered when employing certain kinds or types of synthetic rubbers due to poor tack.
For instance, synthetic rubbers which are saturated or unsaturated polymers of .alpha.-olefin oxides as defined above do not possess good building tack. They are extremely difficult to fabricate into finished rubber goods because of their poor building tack.
On the other hand, a synthetic rubber which is prepared by the polymerization of at least 80 weight percent of at least one C.sub.5 -C.sub.10 .alpha.-olefin with up to 20 weight percent of at least one C.sub.5 -C.sub.30 nonconjugated polyene possesses extremely good building tack but sometimes exhibits a processibility problem.
The term "processibility" is another characteristic of a synthetic rubber and refers to the ability to form a coherent sheet or band on a conventional rubber mill. A synthetic rubber which does not possess good processibility characteristics may crumble and fall off the mill rolls while being compounded with the compounding ingredients necessary for their cure. Such compounded stocks exhibit uneven surfaces or ragged edges because of their lack of good processing characteristics.
Extrudability is another important characteristic of a rubber which is to be employed in the manufacturing operations where rubber must be extruded to form the shaped article prior to curing. A rubber which extrudes poorly will exit the extrusion dye with ragged edges resulting in either rejects or an excessive and costly trimming problem. A standardized method for the evaluation of extrudability is set forth in ASTM D-2230.
The applicants have discovered that their invention provides rubber compositions which in the uncured state exhibits good building tack, good processibility and good extrudability.